A continuous sheet of foamed synthetic, thermoplastic polymer such as polystyrene, polyethylene, or polypropylene, is commonly obtained by an extrusion/expansion process. In this process, an extruder forms, under high pressure, a molten mass of polymer in which a volatile expansion agent, for example a Freon or n-pentane is uniformly dissolved or dispersed, and the mass is extruded through a straight, narrow slit. This results in a decompression which liberates the expansion agent in the form of gas bubbles dispered substantially uniformly in the polymer matrix, which produces two effects:
a substantial increase in the thickness and width of the flow of material leaving the extrusion slit and
cooling of the foamed flow obtained, which in turn produces the increase in viscosity of the polymeric matrix which is necessary to prevent the pressure developed by the expansion agent from causing rupture of the foam cells (and hence collapse of the foam).
Further cooling, applied externally, is necessary to harden the foam. For this purpose, typically, the two faces of the foamed sheet leaving the extruder are guided into contact with cold surfaces, which may diverge from each other slightly in the direction of advancement of the sheet and which limit the expansion of the foam in a direction perpendicular to the plane of the sheet and simultaneously gel the foam. After further cooling (generally with air), the longitudinal edges of the sheet are trimmed and the sheet is cut transversely into panels of a desired length, but the present invention is not concerned with this stage.
For reasons known to the expert in the art, the general trend is to make foamed sheets with the lowest possible density, for example less than 0.05 g/cc and preferably less than 0.03 g/cc. in order to achieve this result, in addition to ensuring the precise metering of the large quantity of expansion agent and an extremely uniform distribution thereof in the molten mass in the extruder, the final temperature of the mass before extrusion must be controlled very accurately in order to raise the viscosity of the mass to a high value, very close to (but not less than) the "critical viscosity". By "critical" viscosity is meant that value of the viscosity below which the pressure of the gas trapped in the foam would rupture the cell walls. Clearly, the higher the viscosity of the mass above the critical viscosity, the smaller is the degree of expansion of the foam and hence the greater the density of the foamed sheet obtained. On the other hand, since the expansion at the outlet from the extruder is accompanied by "endogenic" cooling due to the change in state of the expansion agent from a liquid (or solid) to a gas, the viscosity of the polymeric matrix increases, as a result, and restricts the expansion process even if the pressure of the gas within the matrix is still substantially above atmospheric. It follows that both the density of the foam and the thickness of the sheet obtained are limited correspondingly. It should be noted that the thickness of the sheet cannot be increased at will by widening the extrusion slit appropriately in that, by so doing, the counterpressure exerted by the slit on the molten mass which reaches it would be reduced with consequent premature and detrimental liberation of the expansion agent in the gaseous form within the extruder. This means (inter alia) that it is impossible to obtain very thick foamed sheet with a low-capacity extruder. Even with medium-capacity extruders (200 to 250 kg/hour of material worked) it is difficult to make a sheet which is more than 3 to 4 cm thick.
In order to remedy this disadvantage by some method, it is known to make use of "post-heating" of the previously cooled sheet; this consists of passing the sheet through an oven in which the temperature of the sheet is raised appropriately to give a certain plasticity of the polymeric matrix. Under these conditions the gas trapped in the matrix increases in pressure and is able to enlarge the cells somewhat, with a corresponding increase in the thickness of the sheet and corresponding lowering of the density. However, the increase in thickness obtained in this manner is relatively small, it being difficult to increase it beyond about 10% (in round figures). In order to obtain thicknesses of the order of 6 to 7 cm it is known to make use of the simultaneous extrusion, through two separate slits, of two layers of foam which are superimposed and bonded together immediately downstream of the respective slits. In this case, however, it is extremely difficult to make the two foams identical to each other so that the composite sheet obtained tends to curve or twist. In addition, it is also difficult to achieve complete and effective mutual bonding over the entire interface.
The main object of the present invention is to avoid the disadvantages mentioned above. Further objects and advantages will become evident from the following description.
SUMMARY OF THE INVENTION
The process according to the invention for the production of a continuous sheet of foamed synthetic thermoplastic polymer (particularly polystyrene, polyethylene and polypropylene) by extrusion of a molten mass of the polymer in which a volatile expansion agent is dissolved or dispersed, is characterised by the stages of: (a) producing a progressive increase in the thickness of the foamed sheet during its advance by passing the sheet between two opposing zones which apply a subatmospheric pressure to the respective faces of the sheet while the sheet is in the thermoplastic state and (b) continuing to apply a subatmospheric pressure to the said faces under conditions of constant thickness and cooling the sheet externally to stabilise the thickness achieved.
According to one advantageous embodiment, the thickness of the sheet in the stages (a) and (b) is controlled by gripping the sheet between confining walls which are permeable to gas and applied against the respective faces of the sheet, and applying the subatmospheric pressure through the said walls while the latter are advanced in synchronism with the sheet. This can easily be achieved in practice by means of belt conveyors with gas-permeable belts. In the first pair thereof, in stage (a), the active passes of the conveyors diverge from each other in the direction of advance of the sheet in order to produce the desired, progressive increase in the thickness of the incremental sections of the sheet gripped between these passes. In the second pair, in stage (b), the active passes of the conveyors are parallel to each other in order at least substantially to preserve the increased thickness of the sheet while the latter is cooled.
The stages (a) and (b) may be applied to the sheet during its formation, that is immediately downstream of the extrusion slit. However, since the temperature distribution throughout the thickness of the sheet during its formation is difficult to maintain reliably at a given value and hence varies rapidly with the expansion of the gas, an embodiment is preferred in which the two faces of the foamed sheet leaving the extruder are guided into contact with cold surfaces to produce gelling of the foam, after which, according to the invention:
the sheet, treated in this manner, is passed through a heating zone before the temperature of the core of the sheet has fallen substantially below the incipient softening point of the polymer, and
the heating of the sheet in the heating zone is carried out so as to bring the entire thickness of the sheet to at least substantially the same temperature, between the said incipient softening point and the critical viscosity point (as defined above),
after which the stages (a) and (b) are applied to the sheet.
Preferably the said temperature of the core, expressed in degrees C., is less than the incipient softening point of the polymer and is at least half (preferably about two thirds) of the extrusion temperature. Thus, with this subsequent heating, the desired, uniform distribution of the temperature throughout the thickness of the sheet is achieved more securely.